This proposal seeks funding for continuing studies of the structural and functional embryology of the auditory system, and studies of hair cell regeneration in the avian inner ear. Embryological experiments will be conducted on the brainstem auditory pathways of the chick. They will concentrate on early embryological events involving cellular interactions that may underlie the development of topographic registry between successive regions of the auditory pathways. State of the art microscopic and cell marking methods win be used to investigate cell lineage, migration, and axonal terminal field formation. Functional development will be examined in the inner ear and cochlear nucleus of gerbils. Experiments are designed to examine the development of high frequency hearing, and to investigate relationships between the ontogeny of mature neuronal responses, place-code development, and nonlinear "active" properties of the inner ear. Ontogenetic properties of otoacoustic emissions will be directly compared to neurophysiological response properties. Finally, cellular properties underlying hair cell regeneration will be examined in neonatal chickens. These studies include ultrastructural analyses of support cells in the auditory and vestibular epithelia, further delineation of the progenitor population, and in vitro analyses of molecular signals regulating mitotic activity.